Tapered roller bearing

ABSTRACT

A raceway surface (3a) of an outer ring (3) of a tapered roller bearing (1) includes a composite crowning surface. The composite crowning surface includes a center curve (3a1), which is formed at a center portion in a generating-line direction, and end portion curves (3a2 and 3a3), which are formed on both sides of the center curve (3a1) in the generating-line direction. The raceway surface (3a) of the outer ring (3) is entirely subjected to superfinishing. Each of a ratio (R2/R1) of a curvature radius (R2) of the end portion curve (3a2) to a curvature radius (R1) of the center curve (3a1) and a ratio (R3/R1) of a curvature radius (R3) of the end portion curve (3a3) to the curvature radius (R1) is set to 0.02 or more. Each of drop amounts of the end portion curves (3a2 and 3a3) is set to 0.07 mm or less.

The present application is a U.S. National Stage Application ofInternational (PCT) Application No. PCT/JP2016/050147, filed on Jan. 5,2016, which claims priority to Japanese Patent Application No.2015-014201, filed on Jan. 28, 2015, and Japanese Patent Application No.2015-219472, filed on Nov. 9, 2015.

TECHNICAL FIELD

The present invention relates to a tapered roller bearing.

BACKGROUND ART

Reduction in size of a transmission unit or a differential unit(hereinafter referred to as “transmission unit or the like”) of anautomobile has been demanded for the purpose of reducing fuelconsumption and increasing a cabin space. Along with such demands,reduction in torque and reduction in size have been demanded also for atapered roller bearing which is to be assembled in the transmission unitor the like. In order to achieve the reduction in torque and reductionin size of the tapered roller bearing, increase in load capacity hasbeen demanded. For example, in Patent Literature 1, there is described atapered roller bearing having a roller coefficient (roller fillingratio) which is set to more than 0.94 to improve the load capacity.

In recent years, there has been an increasing demand for reduction insize of the transmission unit or the like, and further increase in loadcapacity has been demanded also for a bearing which is to be assembledin the transmission unit or the like. Further, for the purpose ofreducing the size of the transmission unit or the like, employment of analuminum housing and reduction in thickness of a housing have beenconsidered. In this case, the stiffness of the entire unit is reduced,and a large moment load is applied to the tapered roller bearing.Consequently, a load condition of the tapered roller bearing becomesstricter. Further, when the aluminum housing is employed, the amount ofthermal expansion of the housing is increased, with the result thatreduction in preload (which is so-called “preload loss”) becomes moreliable to occur in the tapered roller bearing. Therefore, more highlyfunctional tapered roller bearing is demanded.

As described above, the demands on the tapered roller bearing havebecome stricter, and there is difficulty in meeting the demands by onlyincreasing the roller coefficient as in Patent Literature 1.

As a method for further increasing the load capacity of the taperedroller bearing, for example, there has been known a method involvingforming each of raceway surfaces of an inner ring and an outer ring intoa crowning shape. For example, in Patent Literature 2, there isdescribed a raceway surface including a so-called composite crowningsurface. In the composite crowning surface, an arcuate curve having alarge curvature radius (large-arc portion) is formed at a center portionof the raceway surface in a generating-line direction, and arcuatecurves each having a small curvature radius (small-arc portions) areformed at both end portions of the raceway surface in thegenerating-line direction. As described above, with the raceway surfaceincluding the composite crowning surface, when the large-arc portionformed at the center of the raceway surface and the roller are broughtinto contact with each other during the normal use, a contact lengthbetween the raceway surface and the roller is increased to reduce asurface pressure, thereby being capable of preventing defects such assurface-originating separation in an early stage. Meanwhile, with thesmall-arc portions formed at the end portions of the raceway surface,the end portions can be recessed toward the side apart from the roller.Therefore, for example, even when a high load is applied to the taperedroller bearing, the contact between the raceway surface of the outerring and the end portions of the tapered roller is avoided as much aspossible, thereby being capable of preventing generation of an excessiveedge load.

CITATION LIST

Patent Literature 1: JP 2005-188738

Patent Literature 2: JP 2007-260829

SUMMARY OF INVENTION Technical Problem

Incidentally, in the tapered roller bearing, a contact surface pressurebetween the raceway surface of the inner ring and the rolling surface ofthe roller is typically higher than a contact surface pressure betweenthe raceway surface of the outer ring and the rolling surface of theroller. Therefore, the above-mentioned raceway surface including thecomposite crowning surface has often been applied to the raceway surfaceof the inner ring which involves a strict load condition, but actuallyhas almost never been applied to the raceway surface of the outer ringwhich involves a relatively moderate load condition.

In order to meet the demands in recent years for further increase inload capacity in the tapered roller bearing as described above, theinventor of the present invention has conducted an investigation onapplication of the composite crowning surface not only to the racewaysurface of the inner ring but also to the raceway surface of the outerring. As a result of the investigation, the following problem has beenfound. In order to avoid the excessive edge load caused by the contactbetween the raceway surface of the outer ring and the end portion of thetapered roller, it has been considered preferable that the curvatureradius of each of the small-arc portions formed at the end portions ofthe raceway surface of the outer ring be set as small as possible, thatis, the curvature be set as large as possible to cause the small-arcportions to be separated from the rolling surface of the tapered rolleras much as possible. However, according to the investigation conductedby the inventor of the present invention, it has been found that, whenthe curvature of each of the small-arc portions at the end portions ofthe raceway surface is set excessively large, the cycle time forsuperfinishing applied to the raceway surface becomes longer, therebyleading to a problem of significant degradation in productivity.

The present invention has an object to improve productivity of a taperedroller bearing in which a composite crowning surface is applied to araceway surface of an outer ring.

Solution to Problem

The raceway surfaces of the outer ring and an inner ring are typicallysubjected to the superfinishing after grinding. The superfinishing isperformed, for example, in the manner as described in Patent Literature2. That is, under a state in which a grinding wheel is pressed againstthe raceway surface of the outer ring or the inner ring being rotated,the grinding wheel is reciprocated along a generating-line direction ofthe raceway surface (see FIG. 7). The inventor of the present inventionperformed the above-mentioned superfinishing on the raceway surface ofthe outer ring which is formed of the composite crowning surface. As aresult, it has been found that the cycle time becomes longer as acurvature of each of end portion curves of the raceway surface islarger. The following cause is conceivable.

During the superfinishing, while the grinding wheel machines the racewaysurface, the grinding wheel itself is also deformed or worn to conformto the raceway surface. As a result, a favorable contact state betweenthe grinding wheel and the raceway surface is obtained, and a machiningefficiency is improved. That is, a large-arc portion formed at thecenter portion of the raceway surface is polished, with the result thatthe grinding wheel is worn to conform to the large-arc portion. Then,small-arc portions at the end portions of the raceway surface arepolished with the grinding wheel, with the result that the grindingwheel is worn to conform to each of the small-arc portions. At thistime, when there is a large difference in curvature between thelarge-arc portion and each of the small-arc portions, it takes a longtime period to cause the grinding wheel, which is worn to conform to thelarge-arc portion, to conform to each of the small-arc portions, withthe result that the machining efficiency during this time period isdegraded. Similarly, it also takes a long time period to cause thegrinding wheel, which is worn to conform to each of the small-arcportions, to conform to the large-arc portion, with the result that themachining efficiency is further degraded.

Based on the findings described above, according to one embodiment ofthe present invention, there is provided a tapered roller bearing,comprising: an inner ring comprising a raceway surface having a taperedshape on an outer peripheral surface of the inner ring; an outer ringcomprising a raceway surface having a tapered shape on an innerperipheral surface of the outer ring; a plurality of tapered rollers,which are arranged between the raceway surface of the inner ring and theraceway surface of the outer ring so as to be rollable and each comprisea rolling surface having a tapered shape on an outer peripheral surfaceof each of the plurality of tapered rollers; and a cage which isconfigured to retain the plurality of tapered rollers at predeterminedintervals, wherein the raceway surface of the outer ring comprises acomposite crowning surface, the composite crowning surface comprising acenter curve, which is formed at a center portion in a generating-linedirection and has a curvature radius R₁, and end portion curves, whichare formed on both sides of the center curve in the generating-linedirection and each have a curvature radius R₂,R₃ which is smaller than acurvature radius R₁ of the center curve, wherein the raceway surface ofthe outer ring is entirely subjected to superfinishing, and wherein eachof a ratio R₂/R₁ of the curvature radius R₂ of the end portion curve tothe curvature radius R₁ of the center curve and a ratio R₃/R₁ of thecurvature radius R₃ of the end portion curve to the curvature radius R₁of the center curve is set to 0.02 or more, and each of drop amounts ofthe end portion curves is set to 0.07 mm or less.

The center curve and the end portion curves are not limited to thearcuate curves, and may comprise non-arcuate curves (for example,logarithmic curves). In the case of the non-arcuate curves, the minimumcurvature radii of the non-arcuate curves are set to curvature radii R₁,R₂, and R₃.

As described above, in the raceway surface of the outer ring, adifference between the curvature radius R₁ of the center curve and eachof the curvature radii R₂ and R₃ of the end portion curves is suppressedwithin a predetermined range, and each of drop amounts of the endportion curves is set to a predetermined amount or less. As a result,the cycle time for the superfinishing is shortened, and hence theproductivity is improved.

It is preferred that, in the above-mentioned tapered roller bearing,each of the drop amounts of both end portions of the raceway surface ofthe outer ring be set to 0.02 mm or more. With this, the end portions(end portion curves) of the raceway surface of the outer ring can besufficiently separated apart from the tapered roller. Therefore, thecontact between each of the corner portions of the tapered roller andthe raceway surface of the outer ring is avoided as much as possible,thereby being capable of reliably preventing generation of the excessiveedge load.

Incidentally, when the thermal expansion occurs in the housing to whichthe outer ring is fixed, the preload having been applied to the taperedroller bearing may be lost, with the result that the outer ring may bemoved relative to the tapered roller toward a small-diameter side in anaxial direction. In this case, there is a fear in that the rollingsurface of the tapered roller overhangs from the raceway surface of theouter ring toward a large-diameter side to cause generation of theexcessive edge load. In particular, the aluminum housing may involve alarge amount of thermal expansion. Therefore, when the outer ring isfixed to an inner peripheral surface of the aluminum housing, theabove-mentioned problem is conspicuous. Also in such a case ofoccurrence of the thermal expansion, when the preload is increased toprevent occurrence of the preload loss, an excessively large load isapplied to the bearing during the normal use, with the result that thelifetime of the bearing is shortened.

In view of the above, it is preferred that a generating-line-directionwidth of the raceway surface of the outer ring with respect to theroller rolling surface be set so that the rolling surface of the taperedroller (hereinafter also referred to as “roller rolling surface”) isprevented from overhanging from the raceway surface of the outer ringtoward the large-diameter side even when occurrence of the preload lossin the tapered roller bearing causes the outer ring to move relative tothe tapered roller in the axial direction. Specifically, it is preferredthat, under a state in which the inner ring, the outer ring, and theplurality of tapered rollers are arranged at normal positions, agenerating-line-direction dimension W₂ (see FIG. 1) of a region of theraceway surface of the outer ring on the large-diameter side from therolling surface of each of the tapered rollers be set in considerationof the amount of relative movement of the outer ring, which is caused bythe preload loss. For example, when the generating-line-directiondimension W₂ of the above-mentioned region is set to 0.6 mm or more, theoverhanging of the roller rolling surface from the outer ring racewaysurface can be prevented even when the preload loss is liable to occur,for example, even when the outer ring is fixed to the aluminum housing.The “state in which the inner ring, the outer ring, and the taperedrollers are arranged at normal positions” corresponds to a state inwhich an appropriate preload in the axial direction is applied to theinner ring and the outer ring.

For example, in the tapered roller bearing configured to mainly receivethe load in a radial direction among bearings configured to support arotary shaft of a transmission which takes a horizontal posture, adownward load is applied to the inner ring. In this case, most of theload is applied to the tapered rollers on the lower half among theplurality of tapered rollers which are arranged at equal intervals in acircumferential direction, whereas the tapered rollers which arearranged near an upper end are brought into a substantially no-loadstate. At this time, the interval between the raceway surfaces of theinner ring and the outer ring sandwiching the tapered rollers near theupper end where substantially no load is applied is slightly increased,and the tapered rollers may be moved relative to the outer ring by theamount of the slight increase in interval toward the small-diameter sidein the axial direction (see the dotted lines in FIG. 6). In such a case,in order to prevent overhanging of the roller rolling surface from theraceway surface of the outer ring toward the small-diameter side, it isnecessary that the raceway surface of the outer ring be extended towardthe small-diameter side from the rolling surfaces of the tapered rollersarranged at the normal positions (see the solid lines in FIG. 6). Atthis time, under the state in which the inner ring, the outer ring, andthe plurality of tapered rollers are arranged at normal positions, adistance W₃ between a small-diameter-side end surface of each of thetapered rollers and a small-flange portion of the inner ring is setsmall. With this configuration, even when the tapered rollers are movedtoward the small-diameter side, the movement of the tapered rollers canbe regulated by causing the tapered rollers to be brought into abutmentagainst the small-flange portion of the inner ring, thereby beingcapable of reducing the generating-line-direction width of the racewaysurface of the outer ring. Specifically, it is preferred that theabove-mentioned distance W₃ be set to 0.4 mm or less.

Incidentally, with regard to the end portion curves of the outer ringraceway surface on the small-diameter side, when the drop amounts are tobe reduced without changing the curvature radii, for example, it isconceivable to move an axial position of the small-diameter-side endsurface of the outer ring toward the large-diameter side, to therebyreduce the width of the end portion curves. However, the interval of thesmall-diameter-side end surface of the outer ring and thelarge-diameter-side end surface of the inner ring in the axial directionis often determined in accordance with a device in which the taperedroller bearing is to be assembled (for example, a transmission or thelike). Therefore, a position of the small-diameter-side end surface ofthe outer ring may not be changed unreasonably. In view of the above,when an axial width W₁ (see FIG. 6) of a chamfered portion formed on asmall-diameter-side end portion of the inner peripheral surface of theouter ring is set large, for example, set to 0.5 mm or more, the widthof the end portion curve can be reduced, thereby being capable ofsetting the drop amount to be small without moving the position of thesmall-diameter-side end surface of the outer ring.

Alternatively, also when a cylindrical surface is formed between thechamfered portion, which is formed on the small-diameter-side endportion of the inner peripheral surface of the outer ring, and theraceway surface, similarly to the above, the width of each of the endportion curves can be reduced, thereby being capable of setting the dropamount to be small without moving the position of thesmall-diameter-side end surface of the outer ring.

Advantageous Effects of Invention

As described above, according to the tapered roller bearing of thepresent invention, the cycle time for superfinishing of the racewaysurface of the outer ring comprising the composite crowning surface isshortened, thereby being capable of improving the productivity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a tapered roller bearing according to oneembodiment of the present invention, which is taken along an axialdirection of the tapered roller bearing.

FIG. 2 is a sectional view of the tapered roller bearing, which is takenalong a direction orthogonal to the axial direction.

FIG. 3 is a side view for illustrating an outer ring raceway surface ofthe tapered roller bearing in an exaggerated manner.

FIG. 4 is a sectional view of another example of the tapered rollerbearing, which is taken along the axial direction.

FIG. 5 is a sectional view of the tapered roller bearing of FIG. 1,which is taken along the axial direction, for illustrating a state inwhich the outer ring of the tapered roller bearing of FIG. 1 is moved inthe axial direction.

FIG. 6 is an enlarged view of the tapered roller bearing of FIG. 1.

FIG. 7 is a side view for illustrating a state in which the racewaysurface of the outer ring is subjected to superfinishing.

DESCRIPTION OF EMBODIMENTS

Now, a tapered roller bearing according to one embodiment of the presentinvention is described with reference to the drawings.

As illustrated in FIG. 1 and FIG. 2, a tapered roller bearing 1 of thisembodiment comprises an inner ring 2, an outer ring 3, a plurality oftapered rollers 4, and a cage 5. The inner ring 2 comprises a racewaysurface 2 a having a tapered shape on an outer peripheral surfacethereof. The outer ring 3 comprises a raceway surface 3 a having atapered shape on an inner peripheral surface thereof. The plurality oftapered rollers 4 are arranged between the raceway surface 2 a of theinner ring 2 and the raceway surface 3 a of the outer ring 3 so as to berollable and each comprise a rolling surface 4 a having a tapered shapeon an outer peripheral surface thereof. The cage 5 is configured toretain the tapered rollers 4 at equal intervals in a circumferentialdirection. The inner ring 2, the outer ring 3, and the tapered rollers 4are each made of a steel material, such as bearing steel, carburizedsteel, or stainless steel. The cage 5 is integrally made of metal orresin. In the following description, a small-diameter side (left side inFIG. 1) of the tapered roller 4 in an axial direction (axial directionof the inner ring 2 and the outer ring 3) is referred to as“small-diameter side”, and a large-diameter side (right side in FIG. 1)of the tapered roller 4 in the axial direction is referred to as“large-diameter side”.

The tapered roller bearing 1 is assembled in, for example, atransmission unit or a differential unit of an automobile. Specifically,as illustrated in FIG. 1, an outer peripheral surface 3 b of the outerring 3 is press-fitted to an inner peripheral surface 10 a of a housing10, and an inner peripheral surface 2 b of the inner ring 2 ispress-fitted to an outer peripheral surface 20 a of a shaft 20. Thehousing 10 is made of, for example, aluminum. The shaft 20 is made of,for example, chrome-molybdenum steel. A shoulder surface 10 b of thehousing 10 and a shoulder surface 20 b of the shaft 20 sandwich andpressurize an end surface 3 c of the outer ring 3 on the small-diameterside and an end surface 2 c of the inner ring 2 on the large-diameterside from both sides in the axial direction, to thereby apply a preloadin the axial direction to the tapered roller bearing 1. In theillustrated example, a shim 30 is interposed between the end surface 3 cof the outer ring 3 on the small-diameter side and the shoulder surface10 b of the housing 10. A thickness of the shim 30 is suitably set sothat a magnitude of the preload to be applied to the tapered rollerbearing 1 is adjusted.

The inner ring 2 has a small-flange portion 2 d and a large-flangeportion 2 e. The small-flange portion 2 d is formed on thesmall-diameter side of the raceway surface 2 a. The large-flange portion2 e is formed on the large-diameter side of the raceway surface 2 a. Theraceway surface 2 a of the inner ring 2 comprises a crowning surfaceformed of a single curve, or a composite crowning surface formed of acenter curve and end portion curves formed on both sides of the centercurve. Each curve is formed of an arc or a logarithmic curve.

As illustrated in FIG. 3, the raceway surface 3 a of the outer ring 3 isformed of a center curve 3 a 1 which is formed at a center portion in agenerating-line direction, an end portion curve 3 a 2 on thesmall-diameter side which is adjacent to the small-diameter side (leftside in FIG. 3) of the center curve 3 a 1, and an end portion curve 3 a3 on the large-diameter side which is adjacent to the large-diameterside (right side in FIG. 3) of the center curve 3 a 1. In thisembodiment, the center curve 3 a 1 comprises an arcuate curve having acurvature radius R₁. The end portion curve 3 a 2 on the small-diameterside comprises an arcuate curve having a curvature radius R₂. The endportion curve 3 a 3 on the large-diameter side comprises an arcuatecurve having a curvature radius R₃. The center curve 3 a 1 and each ofthe end portion curves 3 a 2 and 3 a 3 smoothly continue so as to havecommon tangential lines at borders P₁ and P₂. In FIG. 3, curvatures ofthe curves 3 a 1, 3 a 2, and 3 a 3 of the raceway surface 3 a areillustrated in an exaggerated manner.

In the outer raceway surface 3 a of the outer ring 3, each of a ratioR₂/R₁ of the curvature radius R₂ of the end portion curve 3 a 2 to thecurvature radius R₁ of the center curve 3 a 1 and a ratio R₃/R₁ of thecurvature radius R₃ of the end portion curve 3 a 3 to the curvatureradius R₁ of the center curve 3 a 1 is set to 0.02 or more, preferably0.04 or more. Further, each of the ratio R₂/R₁ and the ratio R₃/R₁ isset to 0.3 or less, preferably 0.1 or less. Each of drop amounts D₁ andD₂ of the end portion curves 3 a 2 and 3 a 3 is set to 0.02 mm or moreand 0.07 mm or less. The drop amounts D₁ and D₂ of the end portioncurves 3 a 2 and 3 a 3 are widths of the end portion curves 3 a 2 and 3a 3 in a direction orthogonal to a generating-line direction of theraceway surface 3 a. Specifically, the generating-line direction of theraceway surface 3 a corresponds to a direction of a straight lineconnecting ends (borders P₁ and P₂) of the center curve 3 a 1.

In this embodiment, an axial width W₁ (see FIG. 6) of a chamferedportion 3 d formed on a small-diameter-side end portion of the innerperipheral surface of the outer ring 3 is set large so that an axialwidth of the end portion curve 3 a 2 on the small-diameter side of theraceway surface 3 a is set small. With this configuration, withoutchanging an axial position of the small-diameter-side end surface 3 c ofthe outer ring 3, the drop amount D₁ of the end portion curve 3 a 2 onthe small-diameter side can be set small. The axial width W₁ of thechamfered portion 3 d is set to, for example, 0.5 mm or more. Further,in order to secure the drop amount D₁ of the end portion curve 3 a 2 onthe small-diameter side to be 0.02 mm or more, the axial width W₁ of thechamfered portion 3 d is set to, for example, 1.0 mm or less.

Instead of or in addition to setting the axial width W₁ of the chamferedportion 3 d to be large, as illustrated in FIG. 4, a cylindrical surface3 f may be formed between the chamfered portion 3 d and the racewaysurface 3 a to suppress the drop amount D₁ of the end portion curve 3 a2 on the small-diameter side.

The rolling surface 4 a of the tapered roller 4 has a substantiallylinear tapered surface, a crowning surface having a single curve, or acomposite crowning surface comprising a center curve and end portioncurves formed on both sides of the center curve. At both ends of theouter peripheral surface of the tapered roller 4, there are formedchamfered portions 4 b and 4 c which are adjacent to the rolling surface4 b.

In the tapered roller bearing 1 of this embodiment, for example, aninner diameter of the inner ring 2 is set within a range of from 15 mmto 120 mm. An outer diameter of the outer ring 3 is set within a rangeof from 30 mm to 250 mm. A combination width, which is an axial widthbetween the small-diameter-side end surface 3 c of the outer ring 3 andthe large-diameter-side end surface 2 c of the inner ring 2, is setwithin a range of from 7 mm to 50 mm. Further, in the tapered rollerbearing 1 of this embodiment, the tapered rollers 4 are loaded at highdensity. Specifically, a roller coefficient γ expressed by the followingexpression is set to γ>0.94.γ=(Z·DA)/(π·PCD)

In the above-mentioned expression, Z represents the number of rollers,DA represents an average diameter of the rollers, and PCD represents apitch circle diameter of the rollers.

The cage 5 comprises a small-diameter side annular portion 5 a, alarge-diameter-side annular portion 5 b, and a plurality of columnarportions 5 c connecting the small-diameter-side annular portion 5 a andthe large-diameter-side annular portion 5 b to each other in the axialdirection (see FIG. 1 and FIG. 2). The cage 5 is arranged on a radiallyouter side of centers of the tapered rollers 4 and at a position of notbeing held in contact with the outer ring 3. Column surfaces 5 d of thecolumnar portions 5 c, which are held in contact with the taperedrollers 4, are inclined so that a gap formed between the column surfaces5 d opposed to each other in the circumferential direction increases asapproaching the radially inner side.

The tapered roller bearing 1 allows the inner ring 2 and the outer ring3 to rotate relative to each other while holding the large-diameter-sideend surfaces 4 e of the tapered rollers 4 in slide contact with thelarge-flange portion 2 e of the inner ring 2. At this time, when preloadloss occurs due to some cause, and the outer ring 3 is moved relative tothe tapered roller 4 toward the small-diameter side (see FIG. 5), thereis a fear in that the rolling surfaces 4 a of the tapered rollers 4 mayoverhang from the raceway surface 3 a of the outer ring 3 toward thelarge-diameter side to cause generation of an excessive edge load.

In view of the above, it is preferred that, under a state in which theinner ring 2, the outer ring 3, and the tapered rollers are arranged atnormal positions (see FIG. 1), a generating-line-direction dimension W₂of a region of the raceway surface 3 a of the outer ring 3 on thelarge-diameter side from the rolling surfaces 4 a of the tapered rollers4, that is, a generating-line direction distance between thelarge-diameter-side end of the rolling surface 4 a of the tapered roller4 and the large-diameter-side end of the raceway surface 3 a of theouter ring 3 be set large in consideration of the preload loss.Particularly in the case where the housing 10 is made of aluminum, thehousing 10 involves a large amount of thermal expansion, and hence thepreload of the tapered roller bearing 1 is liable to be lost. In such acase, it is preferred that the generating-line-direction dimension W₂ ofthe above-mentioned region be set to, for example, 0.6 mm or more.

In this embodiment, a distance W₃ (see FIG. 6) between an end surface 4d of the tapered roller 4 on the small-diameter side and thesmall-flange portion 2 d of the inner ring 2 is set small in advance.Specifically, under the state in which the inner ring 2, the outer ring3, and the tapered rollers 4 are arranged at the normal positions, theabove-mentioned distance W₃ is set to 0.4 mm or less. With thisconfiguration, even when the tapered rollers 4 are moved toward thesmall-diameter side due to some cause, the tapered rollers 4 are broughtinto abutment against the small-flange portion 2 d in an early stage(see the dotted lines in FIG. 6). With this configuration, the amount ofmovement of the tapered rollers 4 toward the small-diameter side can besuppressed. Therefore, without extension of the raceway surface 3 a ofthe outer ring 3 (in particular, the end portion curve 3 a 2 on thesmall-diameter side) toward the small-diameter side, overhanging of theroller surfaces 4 a of the tapered rollers 4 from the raceway surface 3a of the outer ring 3 toward the small-diameter side can be prevented.When the above-mentioned distance W₃ is excessively small, oil is lesslikely to flow in through the gap formed between the end surfaces 4 d ofthe tapered rollers 4 on the small-diameter side and the small-flangeportion 2 d of the inner ring 2. Therefore, it is preferred that theabove-mentioned distance W₃ be set to 0.2 mm or larger.

The raceway surface 2 a of the inner ring 2, the raceway surface 3 a ofthe outer ring 3, and the rolling surfaces 4 a of the tapered rollers 4are subjected to grinding, and thereafter are subjected tosuperfinishing. The superfinishing on the raceway surface 3 a of theouter ring 3 is performed in the following manner. Under a state inwhich the outer ring 3 is rotated about a center axis thereof, asillustrated in FIG. 7, a grinding wheel 6 is pressed against the racewaysurface 3 a and reciprocated along the generating-line direction of theraceway surface 3 a. At this time, while the grinding wheel 6 isreciprocated along the entire raceway surface 3 a, the grinding wheel 6may be finely vibrated in the generating-line direction. The grindingwheel 6 is pressed against the center curve 3 a 1 in such a manner. As aresult, the distal end surface of the grinding wheel 6 is deformed toconform to the center curve 3 a 1, and the center curve 3 a 1 ispolished. Then, the grinding wheel 6 is pressed against the end portioncurves 3 a 2 and 3 a 3, with the result that the distal end surface ofthe grinding wheel 6 is deformed to conform to the end portion curves 3a 2 and 3 a 3, and the end portion curves 3 a 2 and 3 a 3 are polished.

At this time, as described above, the difference between the curvatureradius R₁ of the center curve 3 a 1 and each of the curvature radii R₂and R₃ of the end portion curves 3 a 2 and 3 a 3 is suppressed to berelatively smaller (R₂/R₁≥0.02 and R₃/R₁≥0.02). Therefore, the amount ofdeformation of the distal end of the grinding wheel 6 is relativelysmaller when the grinding wheel 6 moves from the center curve 3 a 1 tothe end portion curves 3 a 2 and 3 a 3 or moves from the end portioncurves 3 a 2 and 3 a 3 to the center curve 3 a 1. With this, thegrinding wheel 6 conforms to the curves 3 a 1, 3 a 2, and 3 a 3 in anearly stage. As a result, machining is performed with high efficiency,and the cycle time for the superfinishing is shortened, therebyimproving the productivity of the tapered roller bearing 1.

The present invention is not limited to the above-mentioned embodiment.For example, in the above-mentioned embodiment, description is made ofthe case where all of the center curve 3 a 1 and the end portion curves3 a 2 and 3 a 3 of the raceway surface 3 a of the outer ring 3 arearcuate curves. However, the present invention is not limited to thiscase. Any or all of those curves may be non-arcuate curves (for example,logarithmic curves). For example, the center curve 3 a 1 may be anarcuate curve, and both end portion curves 3 a 2 and 3 a 3 may belogarithmic curves. Further, the curvature radius R₂ of the end portioncurve 3 a 2 on the small-diameter side and the curvature radius R₃ ofthe end portion curve 3 a 3 on the large-diameter side may be equal toor different from each other.

In order to verify the effect of the present invention, the followingtest was conducted. First, various types of tapered roller bearingshaving different design dimensions (in particular, factors of the outerring raceway surface) were prepared, and were identified as Examples 1to 8 and Comparative Examples 1 to 5. Each tapered roller bearing hadthe same configuration as that of the embodiment illustrated in FIG. 1to FIG. 3, and design dimensions of respective parts are shown in thefollowing Table 1. Research was made on the following Items (1) to (3)for those tapered roller bearings.

TABLE 1 Bearing functional property Design dimensions (1) (2) (3) R₂/R₁Cycle Contact Roller A B L₁ L₂ L₃ R₁ R₂ R₃ R₃/R₁ D₁ D₂ W₁ W₂ W₃ timepressure position Example 1 70 15 9.1 2.6 2.1 3450 100 100 0.029 0.0370.025 0.5 0.8 0.3 ∘ ∘ ∘ Example 2 75 17 11.2 2.8 2.4 5300 150 150 0.0280.029 0.022 0.5 0.67 0.31 ∘ ∘ ∘ Example 3 70 23 15.1 4.3 3.1 9500 200200 0.021 0.050 0.026 0.7 0.7 0.2 ∘ ∘ ∘ Example 4 80 14.8 8.4 3.2 2.52200 100 100 0.045 0.057 0.036 0.5 0.9 0.25 ∘ ∘ ∘ Example 5 69 12.5 7.22.3 1.8 1300 80 80 0.062 0.039 0.025 0.5 0.9 0.3 ∘ ∘ ∘ Example 6 66 12.46.3 2.1 1.7 1000 60 60 0.060 0.043 0.029 0.5 0.8 0.3 ∘ ∘ ∘ Example 7 6612.4 6.3 2.1 1.7 1700 40 40 0.024 0.059 0.039 1.2 0.45 0.7 ∘ ∘ x Example8 70 15 9.1 2.6 2.1 3450 300 300 0.087 0.015 0.010 1.2 0.5 0.5 ∘ x xComparative 70 15 9.1 2.6 2.1 3450 50 50 0.014 0.071 0.047 1.2 0.5 0.5 x∘ x Example 1 Comparative 75 17 11.2 2.8 2.4 5300 50 50 0.009 0.0810.060 1.5 0.3 0.6 x x x Example 2 Comparative 70 23 15.1 4.3 3.1 1430080 80 0.006 0.118 0.062 1 0.5 0.6 x ∘ x Example 3 Comparative 80 14.88.4 3.2 2.5 4400 40 40 0.009 0.131 0.081 1.2 0.45 0.5 x x x Example 4Comparative 69 12.5 7.2 2.3 1.8 2200 40 40 0.018 0.070 0.043 1 0.35 0.5x x x Example 5 A (mm): Outer diameter dimension of the outer ring 3 B(mm): Axial width of the outer ring 3 L₁ (mm): Generating-line-directiondimension of the center curve 3a1 of the raceway surface 3a of the outerring 3 L₂ (mm): Generating-line-direction dimension of the end portioncurve 3a2 on the small-diameter side of the raceway surface 3a of theouter ring 3 L₃ (mm): Generating-line-direction dimension of the endportion curve 3a3 on the large-diameter side of the raceway surface 3aof the outer ring 3 R₁ (mm): Curvature radius of the center curve 3a1 ofthe raceway surface 3a of the outer ring 3 R₂ (mm): Curvature radius ofthe end portion curve 3a2 on the small-diameter side of the racewaysurface 3a of the outer ring 3 R₃ (mm): Curvature radius of the endportion curve 3a3 on the large-diameter side of the raceway surface 3aof the outer ring 3 D₁ (mm): Drop amount of the end portion curve 3a2 onthe small-diameter side of the raceway surface 3a of the outer ring 3 D₂(mm): Drop amount of the end portion curve 3a3 on the large-diameterside of the raceway surface 3a of the outer ring 3 W₁ (mm): Axial widthof the chamfered portion 3d on the small-diameter side of the outer ring3 W₂ (mm): Generating-line-direction dimension of the region of theraceway surface 3a of the outer ring 3 on the large-diameter side fromthe rolling surface 4a of the tapered roller 4 W₃ (mm): Distance betweenthe end surface 4d of the tapered roller 4 on the small-diameter sideand the small-flange portion 2d of the inner ring 2

(1) Cycle Time for Superfinishing of Outer Ring Raceway Surface

The raceway surface of the outer ring of each tapered roller bearing wassubjected to the superfinishing by the method illustrated in FIG. 7, andthe cycle time for the superfinishing was measured. As a determinationcriterion, the mark of “∘” was given when the cycle time was 30 secondsor less, and the mark of “×” was given when the cycle time exceeded 30seconds.

(2) Surface Pressure (Edge Stress)

The inner ring was rotated under a state in which a predeterminedpreload was applied to the inner ring and the outer ring of each taperedroller bearing, and the surface pressure during the rotation wasmeasured. The measurement of the surface pressure was performed bymeasuring the applied load through residual stress measurement with useof an X-ray. The edge portion at which the end portions of the racewaysurfaces of the inner ring and the outer ring and the rollers are heldin contact was subjected to the measurement. As a determinationcriterion, the mark of “∘” was given when the surface pressure was 4,000MPa or less, and the mark of “×” was given when the surface pressureexceeded 4,000 MPa.

(3) Position of Tapered Roller During Preload Loss

The inner ring was rotated under the state in which the predeterminedpreload was applied to the inner ring and the outer ring of the taperedroller bearing, and the presence or absence of a linear edge contactmark, which may be formed at the time of overhanging of the roller fromthe outer ring raceway surface large-diameter side, on the rollerrolling surface radially outer side was checked. When the linear edgecontact mark was formed on the roller large-diameter-side rollingsurface, the mark of “×” was given.

As shown in Table 1, in Examples 1 to 8 in which each of curvatureradius ratios R₂/R₁ and R₃/R₁ of the end portion curves to the centercurve of the outer ring raceway surface was set to 0.02 or more and inwhich each of the drop amounts of the end portion curves was set 0.07 mmor less, the cycle time for the superfinishing of the outer ring racewaysurface was 30 seconds or less. In contrast, in Examples 1 to 5 in whicheach of the curvature radius ratios R₂/R₁ and R₃/R₁ was less than 0.02and in which each of the drop amounts of the end portion curves was morethan 0.07 mm, the cycle time for the superfinishing of the outer ringraceway surface was longer than 30 seconds. Further, in Examples 4, 5,6, and 8 in which each of the curvature radius ratios R₂/R₁ and R₃/R₁was especially larger, that is, was set to 0.04 or more, the cycle timefor the superfinishing was especially shorter.

Further, in Examples 1 to 6, each of the drop amounts of the end portioncurves of the outer ring raceway surface was set to 0.02 mm or more. Thewidth W₂ in the region of the outer ring raceway surface on thelarge-diameter side from the tapered roller was set to 0.6 mm or more.The distance W₃ between the small-flange portion of the inner ring andthe end surface of the tapered roller on the small-diameter side was setto 0.4 mm or less. In Examples 1 to 6, the edge stress was notgenerated, and there was no abnormality in the surface pressuredistribution also during the preload loss. Thus, the tapered rollerbearings of Examples 1 to 6 had sufficient durability.

In contrast, in Example 8, each of the drop amounts of the end portioncurves of the outer ring raceway surface was less than 0.02 mm.Consequently, the excessive edge stress was generated, and the edgeportion was damaged. In Example 7 and Comparative Examples 1 to 5, eachof the drop amounts of the end portion curves of the outer ring racewaysurface was set to 0.02 mm or more. However, the width W₂ of the regionof the outer ring raceway surface on the large-diameter side from thetapered roller was less than 0.6 mm, and the distance W₃ between thesmall-flange portion of the inner ring and the end surface of thetapered roller on the small-diameter side exceeded 0.4 mm. Consequently,during the preload loss, the rolling surface of the tapered rolleroverhung relative to the raceway surface of the outer ring, and the edgeof the end portion of the raceway surface was brought into contact withthe roller rolling surface. As a result, the edge portion of the rollerrolling surface or the raceway surfaces of the inner ring and the outerring were damaged.

REFERENCE SIGNS LIST

-   1 tapered roller bearing-   2 inner ring-   2 a raceway surface-   2 d small-flange portion-   2 e large-flange portion-   3 outer ring-   3 a raceway surface-   3 a 1 center curve-   3 a 2, 3 a 3 end portion curve-   3 d, 3 e chamfered portion-   4 tapered roller-   4 a rolling surface-   5 cage-   10 housing-   20 shaft-   30 shim

The invention claimed is:
 1. A tapered roller bearing, comprising: aninner ring comprising a raceway surface having a tapered shape on anouter peripheral surface of the inner ring; an outer ring comprising araceway surface having a tapered shape on an inner peripheral surface ofthe outer ring; a plurality of tapered rollers, which are arrangedbetween the raceway surface of the inner ring and the raceway surface ofthe outer ring so as to be rollable, each of the plurality of taperedrollers comprising an outer peripheral surface having a rolling surfacewith a tapered shape; and a cage which is configured to retain theplurality of tapered rollers at predetermined intervals, wherein theraceway surface of the outer ring comprises a composite crowningsurface, the composite crowning surface comprising a center curve, whichis formed at a center portion in a generating-line direction and has acurvature radius R₁, and end portion curves, which are formed on bothsides of the center curve in the generating-line direction, one of theend portion curves having a curvature radius R₂ that is smaller than thecurvature radius R₁ of the center curve, and another of the end portioncurves having a curvature radius R₃ that is smaller than the curvatureradius R₁ of the center curve, wherein the raceway surface of the outerring is entirely subjected to superfinishing, and wherein each of aratio R₂/R₁ of the curvature radius R₂ of the one end portion curve tothe curvature radius R₁ of the center curve and a ratio R₃/R₁ of thecurvature radius R₃ of the other end portion curve to the curvatureradius R₁ of the center curve is set to 0.02 or more, and each of dropamounts of the end portion curves is set to 0.02 mm or more and 0.07 mmor less.
 2. The tapered roller bearing according to claim 1, wherein,under a state in which an appropriate preload in an axial direction isapplied to the inner ring and the outer ring, agenerating-line-direction dimension of a region of the raceway surfaceof the outer ring on a large-diameter side from the rolling surfaces ofthe plurality of tapered rollers is set to 0.6 mm or more.
 3. Thetapered roller bearing according to claim 1, wherein the inner ringcomprises a small-flange portion which is formed on a small-diameterside of the raceway surface of the inner ring, and wherein, under astate in which an appropriate preload in an axial direction is appliedto the inner ring and the outer ring, a distance between asmall-diameter-side end surface of each of the plurality of taperedrollers and the small-flange portion of the inner ring is set to 0.4 mmor less.
 4. The tapered roller bearing according to claim 1, wherein theouter ring comprises a chamfered portion formed on a small-diameter-sideend portion of the inner peripheral surface of the outer ring, thechamfered portion having an axial width of 0.5 mm or more.
 5. Thetapered roller bearing according to claim 1, wherein the outer ringcomprises a cylindrical surface formed between a chamfered portion,which is formed on a small-diameter-side end portion of the innerperipheral surface of the outer ring, and the raceway surface of theouter ring.